Is his code buggy or a maintenance disaster waiting to happen? If so then you have a point and an argument. Analyse his code, identify a feature that will require him to rewrite the whole thing but that could be implemented easily in an OO framework. Then ask him to make the change. Point out afterwards how much easier it would have been.

If his code isn't ugly and works, and the thing that is really bothering you is that he wont admire your cool shiny OO framework, then I advise that you get over it. Only if you can really put your finger on things that have gone wrong due to his programming style will you be able to win an argument, and even then it will be a hard battle.